Nucleobase-Functionalized 1,6-Dithiapyrene-Type Electron-Donors: Supramolecular Assemblies by Complementary Hydrogen-Bonds and π-Stacks

Article abstract of DOI:10.1021/cg301414s

Synthesis, crystal structures and redox properties of 1,6-dithiapyrene (DTPY)-type electron-donors functionalized with nucleobases (uracil, cytosine and adenine) were investigated. The electrochemical measurements showed that the uracil-substituted derivatives were slightly stronger electron-donors than DTPY, and the cytosine- and adenine-substitution caused a slight weakening of the electron-donating ability. In the crystal structures, DTPY-nucleobases constructed multidimensional assemblies by complementary hydrogen-bonds on the nucleobase moieties and π-stacks and S...S interactions on the DTPY skeleton. The uracil derivative formed two kinds of hydrogen-bonded pairs with different H-bonding modes (Watson-Crick and reverse Watson-Crick types), both of which were further linked through π-stacks on the DTPY skeleton to construct one-dimensional alternating columns. In the CH₂Cl₂ solvated crystal, the uracil derivative built up a two-dimensional π-layer by the complementary hydrogen-bonds and π-stacks. In the cytosine derivative, the complementary hydrogen-bonded pair assembled by the π-stacks and S...S interactions of the DTPY skeleton constructed a two-dimensional network. The adenine derivative formed a channel structure by the one-dimensional π-stack of complementary hydrogen-bonded pairs, where crystalline water molecules with a ladder-like hydrogen-bonded chain were included. Charge-transfer complexes of DTPY-nucleobases with tetracyanoquinodimethane possessed a neutral ground state and exhibited semiconductive behaviors with room temperature conductivities of 10^-6 to 10^-7 S cm^-1.

Full text of DOI:10.1021/cg301414s

Article
pubs.acs.org/crystal
Nucleobase-Functionalized 1,6-Dithiapyrene-Type Electron-Donors:
Supramolecular Assemblies by Complementary Hydrogen-Bonds
and π‑Stacks
Tsuyoshi Murata, Eigo Miyazaki, Kazuhiro Nakasuji, and Yasushi Morita*
Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
S
* Supporting Information
ABSTRACT: Synthesis, crystal structures and redox proper-
ties of 1,6-dithiapyrene (DTPY)-type electron-donors func-
tionalized with nucleobases (uracil, cytosine and adenine) were
investigated. The electrochemical measurements showed that
the uracil-substituted derivatives were slightly stronger
electron-donors than DTPY, and the cytosine- and adenine-
substitution caused a slight weakening of the electron-donating
ability. In the crystal structures, DTPY-nucleobases con-
structed multidimensional assemblies by complementary
hydrogen-bonds on the nucleobase moieties and π-stacks
and S···S interactions on the DTPY skeleton. The uracil
derivative formed two kinds of hydrogen-bonded pairs with
different H-bonding modes (Watson−Crick and reverse Watson−Crick types), both of which were further linked through π-
stacks on the DTPY skeleton to construct one-dimensional alternating columns. In the CH2Cl2 solvated crystal, the uracil
derivative built up a two-dimensional π-layer by the complementary hydrogen-bonds and π-stacks. In the cytosine derivative, the
complementary hydrogen-bonded pair assembled by the π-stacks and S···S interactions of the DTPY skeleton constructed a two-
dimensional network. The adenine derivative formed a channel structure by the one-dimensional π-stack of complementary
hydrogen-bonded pairs, where crystalline water molecules with a ladder-like hydrogen-bonded chain were included. Charge-
transfer complexes of DTPY-nucleobases with tetracyanoquinodimethane possessed a neutral ground state and exhibited
semiconductive behaviors with room temperature conductivities of 10−6 to 10−7 S cm−1.
stacks.2 Such a self-assembling ability of nucleobases has been
INTRODUCTION
■
utilized in crystal engineering and supramolecular chemistry3
Supramolecular architectures in biomolecular systems such as
and has attracted much attention of materials chemists in the
peptide, enzyme, DNA, etc. have provided much inspiration for
research fields of molecular magnets,4 conductors,5,6 etc.
the development of various functional molecular materials.1
Author: For the exploration of charge-transfer (CT)
Nucleobases, adenine, thymine, guanine and cytosine (Chart
complexes with new functions based on the structural
modification7,8 and cooperation of proton- and electron-
1), play an important role in the formation of double helical
structure of DNA by forming selective complementary
hydrogen-bonded (H-bonded) pairs and one-dimensional π-
transfer,9 we investigated the H-bond incorporated CT
complexes and salts for the exploration of new molecular
conductors with well-defined assembled structures.10,11 In
addition to the structural aspects, our investigation disclosed
that the molecular recognition ability and high polarizability of
H-bonds can modulate electronic structures and physical
properties of CT complexes, demonstrating the new design
strategies for conducting CT complexes.10a,d We also designed
and synthesized tetrathiafulvalene (TTF) derivatives function-
alized with nucleobases to develop organic conductors
incorporated into biological self-assemblies.12−14 The comple-
mentary H-bonds inherent in nucleobases controlled the
molecular arrangement to construct diverse supramolecular
Chart 1. Chemical Structures of Nucleobases and Electron-
Donor and -Acceptors in the Text
Received: September 27, 2012
Revised: October 19, 2012
Published: October 22, 2012
© 2012 American Chemical Society
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architectures and yielded highly conductive CT complexes.
Furthermore, the chemical modification on TTF-uracil
derivatives, where the terminal vinyl group of TTF skeleton
was substituted with ethylenedithio-15 or ethylenedioxy-
group,16 highly affected the self-assembling ability of the TTF
skeleton causing the increment of variation of molecular
packing.
1,6-Dithiapyrene (DTPY) is a Weitz-type electron-donor
molecule17,18 having an electron-donating ability comparable to
that of TTF (Eox = −0.04 for DTPY and −0.08 V for TTF vs
Fc/Fc+) and an 18π-electronic system larger than that of TTF
(14π). The planar and extensive π-electronic system of DTPY
often induces the formation of a one-dimensional column by
strong π-stacks, being advantageous for a path for electrical
conduction in CT complexes and salts. DTPY and its
derivatives afforded CT complexes exhibiting metallic electrical
conduction.17,18 In the present study, we have designed and
synthesized DTPY derivatives substituted with nucleobase
moieties (uracil, cytosine and adenine, 1−3 in Chart 2) and
investigated their electrochemical behavior, crystal structures
and CT complexes with tetracyanoquinodimethane (TCNQ).
In the crystal structures, the cooperation of complementary H-
bonds of nucleobases and strong π-stacks of DTPY moieties
modulated the self-assembled structures.
EXPERIMENTAL SECTION
■
Materials and Methods. The syntheses of DTPY,19 5-iodouracil
derivatives 5,12 15 1-n-butyl-5-iodo-4-(o-nitrophenoxyl)pyrimidine-2-
a,
one 6,13 8-iodoadenosine derivative 7b14 are presented in our previous
paper, and 9-n-butyl-8-iodoadenine (7a) was newly prepared by the
literature method (see Supporting Information).20 TCNQ was
purchased and purified by sublimation. Solvents were dried (drying
agent in parentheses) and distilled under argon prior to use: THF
(Na−benzophenone ketyl), toluene, DMF and CH2Cl2 (CaH2). All
reactions requiring anhydrous conditions were conducted under an
argon atmosphere. Rf values on TLC were recorded on E. Merck
precoated (0.25 mm) silica gel 60 F254 plates. The plates were sprayed
with a solution of 10% phosphomolybdic acid in 95% EtOH, and then
heated until the spots became clearly visible. Silica gel 60 (100−200
mesh) deactivated by mixing with 6% water was used for column
chromatography.
Chart 2. DTPY-Nucleobase Derivatives
Measurements. Melting points were measured with a hot-stage
1
apparatus, Yanaco MP-J1, and uncorrected. H NMR spectra were
obtained on a JEOL EX-270 with CDCl3 and DMSO-d6 using Me4Si
as an internal standard. Infrared spectra (IR) were recorded on JASCO
FT/IR-660M or Perkin-Elmer 1600 series using KBr plates (resolution
2 or 4 cm−1). Electronic spectra were measured by a Shimadzu UV-
3100PC spectrometer using KBr plates. EI-Mass spectra were taken at
70 eV by using a Shimadzu QP-5000 mass spectrometer, and FAB-MS
spectra were recorded on a Shimadzu/KRATOS Concept 1S using 3-
nitrobenzylalcohol matrix. Elemental analyses were performed at the
Graduate School of Science, Osaka University. Cyclic voltammetric
measurements were made with an ALS Electrochemical Analyzer
Model 612A. Cyclic voltammograms were recorded with a 1.6 mm
diameter gold working electrode and a Pt wire counter electrode for 1
mM solutions of 1−3 in MeCN containing 0.1 M Et4NClO4 as the
supporting electrolyte at room temperature. The experiments
employed an Ag/AgNO3 reference electrode, and the final results
Table 1. Crystallographic Data of DTPY-Nucleobases
1a
1a·CH2Cl2
2a
3a·2H2O
crystal formula
formula weight
crystal system
space group
a (Å)
C22H18N2O2S2
406.50
C23H20Cl2N2O2S2
491.43
C22H19N3O1S2
405.52
C23H23N5O2S2
465.58
triclinic
monoclinic
P21/n
monoclinic
P21/n
monoclinic
P21/c
P1
̅
12.2288(2)
12.4347(3)
13.512(1)
92.751(6)
109.419(8)
106.704(7)
1832.4(1)
4
11.11(2)
8.731(7)
23.51(2)
90
12.5214(9)
9.9250(3)
16.990(1)
90
20.0919(7)
4.8285(1)
22.403(2)
90
b (Å)
c (Å)
α (deg)
β (deg)
98.59(5)
90
111.766(8)
90
101.226(7)
90
γ (deg)
V (Å3)
2254(4)
4
1960.9(2)
4
2131.8(2)
4
Z
Dcalc (g cm−3
)
1.474
1.448
1.374
1.451
a
b
a
a
μ (mm−1
)
2.812
0.497
2.602
2.532
temp (K)
150
150
150
200
unique reflns
6509
3233
532
4795
1559
280
3585
2776
253
3897
1914
305
reflns used (I > 2.0σ(I))
parameters
R1 (I > 2.0σ(I))
wR2 (all data)
GOF
0.0818
0.2648
0.983
0.1262
0.3777
0.985
0.0634
0.1875
1.054
0.0592
0.1251
0.920
a
b
Cu Kα radiation. Mo Kα radiation.
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were calibrated with a ferrocene/ferrocenium (Fc/Fc+) couple.
Temperature dependences of conductivity for compressed pellets
were recorded by a two-probe method on a Fuso dc multichannel
conductivity device HECS 944C type in a temperature range of 200−
300 K.
Information). The observed oxidation potentials are summar-
ized in Table 2. The uracil substituted derivatives 1a and 1b
Table 2. Redox Potentials (E) and Potential Gap (ΔE) of
a
DTPY-Nucleobases
X-ray Crystallographic Analysis. X-ray crystallographic measure-
ments were made on a Rigaku Raxis-Rapid imaging plate with graphite
monochromated Mo Kα (λ = 0.71075 Å) or Cu Kα (λ = 1.54187 Å)
radiation. Empirical absorption corrections were applied. Structures
were determined by a direct method using SHELXS-9721 or SIR-
2004.22 Refinements were performed by full-matrix least-squares on F2
using SHELXL-97.23 All non-hydrogen atoms were refined anisotropi-
cally. Positional parameters of hydrogen atoms were calculated with
sp2 or sp3 configuration of the bonding carbon and nitrogen atoms,
and hydrogen atoms were refined using the riding model. In the
refinement procedures, isotropic temperature factors with magnitudes
of 1.2 times of the equivalent temperature factors of the bonding
atoms were applied for hydrogen atoms. In the cases of 2a, S1−C2 and
S2−C14 disordered due to the reversal of DTPY skeleton. The site-
occupancy factors of 0.875 and 1.333 were applied for S and C atoms,
respectively, which corresponds to the 8:2 probability of each
conformation. Selected crystal data and data collection parameters
are given in Table 1.
Eox1 (V)
Eox2 (V)
ΔE (V)
1a
−0.05
−0.06
+0.02
+0.01
+0.05
+0.08
−0.04
b
b
1b
+0.21
+0.29
+0.28
b
0.27
0.27
0.27
b
2a
2b
3a
3b
b
b
DTPY
+0.27
0.31
a
Conditions: concentration of analyte, 1 mM; solvent, DMF (0.1 M
Et4NClO4); scan rate, 10−100 mV/s; reference electrode, Ag/AgNO3
(0.01 M in MeCN); counter electrode, Pt wire; working electrode, Au
b
disk; the results were calibrated with Fc/Fc+ couple. Irreversible
waves.
show slightly more negative oxidation potentials (Eox1 = −0.05
and −0.06 V, respectively, vs Fc/Fc+) than that of DTPY (Eox1
= −0.04 V), indicating the electron-rich feature of uracil moiety.
The cytosine substitution causes positive shifts of the oxidation
potentials by 0.05−0.06 V for Eox1. Because of the electron-
withdrawing feature of imino-skeleton in the purine-base,
DTPY-adenine derivatives 3 exhibit the oxidation waves at the
highest potentials (Eox1 = +0.05 and +0.08 V for 3a and 3b,
respectively) among DTPY-nucleobases in the present study.
The difference of the first and second oxidation potentials (ΔE
= Eox2 − Eox1), a simple indicator of on-site Coulomb repulsion,
slightly decreased due to the π-extension. These effects of
substitution with nucleobases on the electron-donation abilities
of electron-donors are very similar to those observed in the
TTF-nucleobases.12−16
RESULTS AND DISCUSSION
Synthesis. Scheme 1 presents the synthetic procedures of
DTPY-nucleobases. The key reaction is the Stille-type cross
■
a
Scheme 1. Synthetic Procedures of DTPY-Nucleobases 1−3
Crystal Structure of 1a. Single crystals of 1a suitable for
the X-ray crystal structure analysis were obtained by the vapor
diffusion method using ethyl acetate and hexane. The
asymmetric unit is composed of two 1a molecules (1a-A and
1a-B). In the molecules of 1a-A and 1a-B, the DTPY and
nucleobase skeletons are twisted by 4.4 and 14.5°, respectively,
from each other. Notably, both molecules form pairs with
different kinds of complementary H-bonds: Watson−Crick H-
bonds (N···O distance, 2.96 Å) for 1a-A pair and reverse
Watson−Crick type (2.87 Å) for 1a-B pair (Figure 1, panels a
and b, respectively). Such a coexistence of two kinds of
complementary H-bonds in one crystal structure has been
occasionally reported in uracil derivatives.24 Both 1a molecules
stack in a head-to-tail manner (interplanar distance: 1a-A−1a-
A; 3.54 Å, 1a-B−1a-B; 3.56 Å, respectively), connecting the H-
bonded dimers to form one-dimensional alternating columns
(Figure 1a,b). The π-stack dimers of 1a-A and 1a-B further
stack alternately in a head-to-tail manner with a interplanar
distance of 3.28−4.11 Å and tilt angle of 18.9°, forming a one-
a
Reagents and conditions: (a) n-BuLi, THF, −78 °C; Bu3SnCl, −78
°C to rt, quantitative yield; (b) 10 mol % Pd(PPh3)4, 10 or 30 mol %
CuI, toluene (reflux) or DMF (70 °C), 46% for 1a, 69% for 1b, 54%
for 8, 20% for 3a and 30% for 3b; (c) NH3 aq or n-BuNH2, THF, rt,
57% for both 2a and 2b.
coupling reaction between iodo-nucleobase derivatives 5−7 and
tributylstannylated DTPY which was prepared by the lithiation
of DTPY with n-BuLi followed by the treatment with Bu3SnCl.
The cytosine derivatives 2a and 2b were prepared by the
substituent exchange reaction of the o-nitrophenoxyl group of 8
with amino groups by the treatment with ammonia and n-
butylamine, respectively. All these compounds are stable in air
at room temperature. Because of the protection of N−H
groups, they are soluble to common organic solvents (CH2Cl2,
MeCN, THF, alcohols, etc.).
Electrochemical Properties. Redox properties of DTPY-
nucleobases were evaluated by cyclic voltammetry in DMF at
room temperature. All electron-donors exhibit two-stage one-
electron oxidation waves of the DTPY skeleton, although the
second process (Eox2) for 1a, 3a and 3b is irreversible due to
the insolubility of the dication species (Figure S1, Supporting
dimensional (1a-A)2−(1a-B)2 column along the [011]
̅
direction (Figure 1c,d). It should be noted that, in the π-stacks
of 1a-A−1a-A, 1a-B−1a-B and 1a-A−1a-B (Figure 1a−c), the
electron-rich DTPY skeleton is placed over the electron-poor
uracil moiety suggesting the importance of donor−acceptor
interaction between DTPY and uracil moieties for the
stabilization of these π-stacking structures.
Crystal Structure of 1a·CH2Cl2. The vapor diffusion of 1a
using CH2Cl2 and hexane afforded the CH2Cl2 solvated
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Figure 1. Crystal structure of 1a. (a and b) One-dimensional alternating columns of 1a-A and 1a-B formed by Watson−Crick and reverse Watson−
Crick type H-bonds and π-stacks. (c) Overlap pattern in the 1a-A and 1a-B stack. (d) (1a-A)2−(1a-B)2 alternating column. Red lines show
complementary H-bonds, and two-way arrows present the π-stacks.
Figure 2. Crystal structure of 1a·CH2Cl2. (a) Molecular packing viewed along the c-axis at c = 0 showing the two-dimensional π-layer by π-stacks and
reverse Watson−Crick type H-bonds. Butyl groups are omitted for clarity. Red lines show complementary H-bonds, and two-way arrows present the
π-stacks. (b) Overlap pattern within the π-layer. (c) The b-axis projection of molecular packing showing the alternating stack of the π-layer and Bu−
CH2Cl2 layer.
crystals. One 1a and CH2Cl2 molecules are crystallographically
independent within a unit cell. The twist angle of DTPY and
uracil moieties is 2.2°, and 1a had a planar structure. Differently
from the nonsolvated crystal, only the reverse Watson−Crick
type H-bonds (N···O distance, 2.84 Å) are observed, and 1a
molecules form a H-bonded pair (Figure 2a). π-Stacks of the
pairs (interplanar distance: 3.35−3.56 Å) establish a two-
dimensional layer parallel to the ab-plane (Figure 2b). Similarly
to the crystal structure of nonsolvated 1a (Figure 1a−c), the
stacking between electron-rich DTPY and electron-poor uracil
moieties is found in the π-layer (Figure 2b). The π-layer and
hydrophobic layer comprised of the n-butyl group and CH2Cl2
are arranged alternately along the c-axis (Figure 2c).
CO Stretching Modes in Vibration Spectra. Figure 3
illustrates the CO stretching modes (νCO) in the IR spectra
of 1a, 1a·CH2Cl2 and 1b. The νCO frequency has been
regarded as a useful probe to identify the complementary H-
bonding modes of uracil derivatives.25 TTF-n-butyluracil
derivative in the solution state (H-bond free) shows two
Figure 3. IR spectra of 1a·CH2Cl2, nonsolvated 1a and 1b in the C
O stretching region.
νCO modes at 1716 and 1693 cm−1 (νCO(2) and νCO(4),
respectively).12a A Watson−Crick type H-bonded pair in which
the C4O group participates in the complementary H-bonds,
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Figure 4. Crystal structure of 2a. (a) One-dimensional alternating column of 2a formed by complementary H-bonds. (b) Overlap pattern in a π-
stack dimer. (c) The c-axis projection of molecular packing showing a two-dimensional structure at c = 0. Red and green lines show complementary
H-bonds and S···S contacts, respectively. Butyl groups are omitted for clarity in (c). Two-way arrows in (a) present the π-stacks, and gray two-way
arrows in (c) show the edge-to-face interactions between DTPY and cytosine moieties.
Figure 5. Crystal structure of 3a·2H2O. (a) Molecular packing viewed along the b-axis showing the one-dimensional channel including water
molecules. (b) Overlap pattern within the π-stacking column. (c) The π-stacking column and one-dimensional chain of water molecules. DTPY
skeleton is shown in half-tone colors. Red thick and thin lines show Watson−Crick type H-bonds of the adenine moieties and H-bonds of water
molecules, respectively.
the C4O bond softens to cause a low-frequency shift of
νCO(4) band. The νCO(2) and νCO(4) modes of TTF-n-
butyluracil forming the Watson−Crick type pairs are observed
at 1702 and 1655 cm−1, respectively. In the case of TTF-
phenyluracil derivative which forms the reverse Watson−Crick
type pairs, the νCO(2) mode shifts to a lower frequency region
and appears at the same position of the νCO(4) mode (1697
cm−1).12c
The 1a·CH2Cl2 crystal, exhibiting reverse Watson−Crick
type H-bonds, shows a single band at 1684 cm−1 similar to that
observed in TTF-phenyluracil. The nonsolvated 1a crystal
shows three νCO modes at 1715, 1684, and 1660 cm−1,
suggesting the coexistence of Watson−Crick and reverse
Watson−Crick type H-bonds, well coinciding with the crystal
structure. In the case of 1b, a single νCO mode was observed
at 1714 cm−1 with a tailing down to ∼1650 cm−1 differently
from the features of reverse Watson−Crick H-bonds.
It has been known that the stabilization energies of Watson−
Crick and reversed Watson−Crick type pairs of uracil are close
to each other.26 In our previous studies on TTF-uracil
derivatives, the complementary H-bond types are modulated
by the subtle conditions such as substituent groups at the N1
atom,12c redox state of the TTF moiety12b and crystallization
conditions.16 In the case of 1a, inclusion of CH2Cl2 highly
affected the overall molecular packing and also two
complementary H-bond types.
Crystal Structure of 2a. Single crystals of 2a suitable for X-
ray crystal structure analysis were prepared by vapor diffusion
using toluene and Et2O. The asymmetric unit is composed of
one 2a molecule, in which DTPY and cytosine moieties are
nearly perpendicular to each other (twist angle, 79.2°). The
N2−H···N3 double complementary H-bonds (N···N distance,
2.96 Å, Figure 4a) link the cytosine moieties to form a H-
bonded pair. The similar H-bonded motif is observed in the
TTF-cytosine system,13 suggesting the robustness of this H-
bonding motif. The DTPY skeleton also forms a π-dimer
through the small overlap (Figure 4b, interplanar distance
=3.47 Å), and these interactions construct a one-dimensional
alternating column (Figure 4a). Unlikely to 1a crystals, the
cytosine moiety does not participate in the π-stacking motif
probably due to the large torsion angle between DTPY and
cytosine moieties, while the short edge-to-face C−H···π
interactions (2.60 Å) are formed between electron-rich DTPY
and electron-poor uracil moieties (gray arrows in Figure 4c).
The S1···S1 interaction between adjacent DTPY skeletons
(3.68 Å, slightly longer than the sum of van der Waals radius of
S atoms, 3.60 Å27) links the alternating columns to build up a
two-dimensional sheet parallel to the ab-plane (Figure 4c).
Crystal Structure of 3a·2H2O. Single crystals of 3a
suitable for the X-ray crystal structure analysis were prepared
by the vapor diffusion using DMF and water. The asymmetric
unit is composed of one 3a and two water molecules. DTPY
and adenine moieties are twisted by 18.0° to each other. The
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complementary N5−H···N4 H-bonds on the adenine moieties
form a H-bonded pair of the Watson−Crick type (N···N
distance, 2.97 Å, Figure 5a). The 3a molecules further stack
with a slip-stack fashion (Figure 5b, interplanar distance, 3.56
Å) to build up a uniform one-dimensional column along the b-
axis (Figure 5c). Within a π-stack, adjacent DTPY skeletons are
stacked to each other, and a small overlap between DTPY and
adenine moieties are also found (Figure 5b). The crystalline
water molecules form a one-dimensional ladder-like chain
parallel to the π-stack column of 3a (Figure 5c, O···O distances,
2.79, 2.81, and 2.93 Å). The one-dimensional structures of 3a
and water molecules are further linked through N2···H−O1 H-
bonds (Figure 5c, N···O distance, 2.90 Å).
Author: In the crystal structure of TTF-adenine derivative,14
a one-dimensional ribbon structure by the Hoogsteen type H-
bonds is formed (Scheme 2), where π-stacks between adjacent
Scheme 2. Watson−Crick and Hoogsteen Type H-bonded
Motifs of Adenine
Figure 6. IR spectra (KBr pellet) of TCNQ complexes of 1−3 and
related compounds in the CN stretching region.
neutral TCNQ and TCNQ radical anion species show b1u C
N stretching mode at 2225 and 2195 cm−1, respectively. In the
TCNQ complexes of 1−3, the bands are observed as weak
shoulder peaks around 2207−2210 cm−1, from which ρ of
TCNQ moieties are estimated to be −0.39 to −0.45.28 In the
solid state electronic spectra (Figure 7), low energy absorption
TTF skeletons are not found; however, that between electron-
rich TTF and electron-poor adenine moieties is formed. The
theoretical calculation indicates that the Watson−Crick and
Hoogsteen type H-bonded pairs of adenine possess similar
stabilization energies to each other.26 The stronger π-stacking
ability of the DTPY skeleton to form a one-dimensional
column probably controls the molecular packing and also
modulates the H-bonding motif.
Charge-Transfer Complexes with TCNQ. CT complexes
were prepared by mixing the solutions of 1−3 and TCNQ. The
selected physical properties of TCNQ complexes are
summarized in Table 3. D−A ratios of the TCNQ complexes
of 1a, 2a, 2b and 3a estimated by elemental analysis are 1:1,
3:2, 4:3 and 2:1, respectively.
Figure 6 compares CN stretching modes in the IR spectra
of TCNQ complexes with those of neutral and radical anion
species of TCNQ. The b1u CN stretching frequency of
TCNQ derivatives (νCN) is known to be sensitive to ionicity of
TCNQ (ρ) and is used as a probe for the estimation.28 The
Figure 7. Electronic spectra of TCNQ complexes of 1−3 in KBr
pellet. Arrows indicate the intermolecular CT absorption bands.
a
Table 3. Properties of TCNQ Complexes of 1−3
bands assignable to the intermolecular CT between electron-
donor and -acceptor molecules are observed at 5000−6000
cm−1.29 These observations indicate that the complexes are
weak CT complexes having a neutral ground state (0 < |ρ| <
0.5) in agreement with the estimation of ρ values using CN
stretching frequencies. In the electrical conductivity measure-
ments using compressed pellets, although the individual
components of the complexes, 1−3 and TCNQ, are insulators
(room temperature conductivity (σRT) < 10−8 S cm−1), the
complexes show semiconducting behaviors with σRT of 10−6 to
10−7 S cm−1.
b
c
d
νCN/cm−1 [ρ]
hvCT /cm−1 σRT /cm−1 [εa/meV]
(1a)(TCNQ)
(2a)3(TCNQ)2
(2b)4(TCNQ)3
(3a)2(TCNQ)
2208 [−0.43]
2210 [−0.39]
2207 [−0.45]
2207 [−0.45]
6000
5300
6000
6200
2.3 × 10−6 [337]
2.5 × 10−7 [312]
3.7 × 10−6 [204]
3.4 × 10−7 [ ]
e
a
Molar ratios were estimated from the elemental analyses. Some
complexes contained crystal water or crystal solvent. ρ values are
b
estimated from CN stretching frequency in the IR spectra by the
Chappell’s method.28 The lowest energy CT absorption bands in KBr
c
pellets.29 Electrical conductivity was measured by the two-probe
d
e
method on a compressed pellet. Not measured due to too high
resistivity.
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Crystal Growth & Design
Article
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CONCLUSION
■
We have synthesized new H-bonded electron-donors, where
DTPY was functionalized with nucleobases, uracil, cytosine and
adenine. The combination of complementary H-bonds of
nucleobases and π-stacks on the DTPY skeleton constructed
diverse supramolecular architectures. These observations
indicate that the self-assembling ability of nucleobases and
strong π-stacks of DTPY moiety are effective tools for the
control of molecular arrangement in the future development of
conducting CT complexes based on biological molecular
systems. Furthermore, the present result serves as primary
and important information in the development of novel
molecular architectures with cooperative proton- and elec-
tron-transfer (PET) phenomena,9 for which the exploration of
H-bonded CT complexes and salts is the most essential
strategy.
(7) Comprehensive overview of H-bonded CT complexes and salts,
see: Fourmigue,
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ASSOCIATED CONTENT
* Supporting Information
Synthetic procedures and properties for identification and cyclic
voltammograms of 1−3 in PDF format and X-ray crystallo-
graphic data for each structure in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
(9) (a) Nakasuji, K.; Sugiura, K.; Kitagawa, T.; Toyoda, J.; Okamoto,
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AUTHOR INFORMATION
Corresponding Author
*Address: Department of Chemistry, Graduate School of
Science, Osaka University, 1-1 Machikaneyama, Toyonaka,
Osaka 560-0043, Japan. Phone: +81-6-6850-5393. Fax: +81-6-
6850-5395. E-mail: morita@chem.sci.osaka-u.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partly supported by Grants-in-Aid for Scientific
Research (23750039) from the Japan Society for the
Promotion of Science, and for Scientific Research on Innovative
Area (20110006) and Elements Science and Technology
Project from the Ministry of Education, Culture, Sports,
Sciences and Technology, Japan.
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